[135.04] Dynamical Constraints on the Disk and Spheroid of the Andromeda Spiral Galaxy from Keck Spectroscopy

We present spectroscopy of M31 red giant branch (RGB) stars
acquired with DEIMOS and LRIS on the Keck telescopes. Using
a combination of photometric/spectroscopic diagnostics, we
isolate bona fide M31 RGB stars from the foreground Galactic
dwarf star population. We use kinematics to directly measure
the spheroid/disk ratio, constrain models of the surface
brightness profiles of M31's bulge, disk, and halo, and
directly test the claim of a dominant disk in M31. In
addition, we place limits on overall spheroid rotation along
the major and minor axes of M31. Two major-axis fields,
located at projected distances of 34 kpc SW and 15 kpc NE of
M31's center, show clear evidence for a cold disk-like
population, yet also demonstrate that a significant fraction
of the stars in each of these fields belong to a hot,
non-rotating spheroid-like population. The metallicity of
the stars in each field is derived by interpolating their
location in the (I, V-I) color-magnitude diagram with a
finely spaced grid of theoretical isochrones. The
metallicity distribution function of the disk and spheroid
populations both contain metal-rich stars. Two SE minor-axis
fields located at projected distances of 12 and 30 kpc show
no evidence for a large stellar component in a cold disk,
placing upper limits on the contribution of M31's disk at
large radii.

P.G. acknowledges support from NSF grant AST-0307966. Data
presented herein were obtained at the W.M. Keck Observatory,
which is operated as a scientific partnership among Caltech,
the University of California and NASA. The Observatory was
made possible by the generous financial support of the W.M.
Keck Foundation.